How to Change Mocked Functions For Different Test Scenarios
In my scenario I tried to define the mock function outside of the jest.mock which will return an error about trying to access the variable before it's defined. This is because modern Jest will hoist jest.mock so that it can occur before imports. Unfortunately this leaves you with const and let not functioning as one would expect since the code hoists above your variable definition. Some folks say to use var instead as it would become hoisted, but most linters will yell at you, so as to avoid that hack this is what I came up with:
Jest Deferred Mocked Import Instance Calls Example
This allows us to handle cases like new S3Client() so that all new instances are mocked, but also while mocking out the implementation. You could likely use something like jest-mock-extended here to fully mock out the implementation if you wanted, rather than explicitly define the mock.
The Problem
This example will return the following error:
eferenceError: Cannot access 'getSignedUrlMock' before initialization
Test File
const sendMock = jest.fn()
const getSignedUrlMock = jest.fn().mockResolvedValue('signedUrl')
jest.mock('@aws-sdk/client-s3', () => {
return {
S3Client: jest.fn().mockImplementation(() => ({
send: sendMock.mockResolvedValue('file'),
})),
GetObjectCommand: jest.fn().mockImplementation(() => ({})),
}
})
jest.mock('@aws-sdk/s3-request-presigner', () => {
return {
getSignedUrl: getSignedUrlMock,
}
})
The Answer
You must defer the call in a callback like so:
getSignedUrl: jest.fn().mockImplementation(() => getSignedUrlMock())
Full Example
I don't want to leave anything up to the imagination, although I phaked the some-s3-consumer from the actual project, but it's not too far off.
Test File
import { GetObjectCommand, S3Client } from '@aws-sdk/client-s3'
import { SomeS3Consumer } from './some-s3-consumer'
const sendMock = jest.fn()
const getSignedUrlMock = jest.fn().mockResolvedValue('signedUrl')
jest.mock('@aws-sdk/client-s3', () => {
return {
S3Client: jest.fn().mockImplementation(() => ({
send: sendMock.mockResolvedValue('file'),
})),
GetObjectCommand: jest.fn().mockImplementation(() => ({})),
}
})
jest.mock('@aws-sdk/s3-request-presigner', () => {
return {
// This is weird due to hoisting shenanigans
getSignedUrl: jest.fn().mockImplementation(() => getSignedUrlMock()),
}
})
describe('S3Service', () => {
const service = new SomeS3Consumer()
describe('S3 Client Configuration', () => {
it('creates a new S3Client with expected region and credentials', () => {
expect(S3Client).toHaveBeenCalledWith({
region: 'AWS_REGION',
credentials: {
accessKeyId: 'AWS_ACCESS_KEY_ID',
secretAccessKey: 'AWS_SECRET_ACCESS_KEY',
},
})
})
})
describe('#fileExists', () => {
describe('file exists', () => {
it('returns true', () => {
expect(service.fileExists('bucket', 'key')).resolves.toBe(true)
})
it('calls S3Client.send with GetObjectCommand', async () => {
await service.fileExists('bucket', 'key')
expect(GetObjectCommand).toHaveBeenCalledWith({
Bucket: 'bucket',
Key: 'key',
})
})
})
describe('file does not exist', () => {
beforeEach(() => {
sendMock.mockRejectedValue(new Error('file does not exist'))
})
afterAll(() => {
sendMock.mockResolvedValue('file')
})
it('returns false', async () => {
const response = await service.fileExists('bucket', 'key')
expect(response).toBe(false)
})
})
})
describe('#getSignedUrl', () => {
it('calls GetObjectCommand with correct bucket and key', async () => {
await service.getSignedUrl('bucket', 'key')
expect(GetObjectCommand).toHaveBeenCalledWith({
Bucket: 'bucket',
Key: 'key',
})
})
describe('file exists', () => {
it('returns the signed url', async () => {
const response = await service.getSignedUrl('bucket', 'key')
expect(response).toEqual(ok('signedUrl'))
})
})
describe('file does not exist', () => {
beforeEach(() => {
getSignedUrlMock.mockRejectedValue('file does not exist')
})
afterAll(() => {
sendMock.mockResolvedValue('file')
})
it('returns an S3ErrorGettingSignedUrl with expected error message', async () => {
const response = await service.getSignedUrl('bucket', 'key')
expect(response.val).toStrictEqual(new S3ErrorGettingSignedUrl('file does not exist'))
})
})
})
})
